Change the default configuration file name.

The default configuration file name has been moved from '.pgenv.conf'
to '.pgenv.default.conf'.
This is a more rational name and keeps simpler the juggling between the
functions that internally try to guess the PostgreSQL versions.
However, **this breaks backward compatibility** with versions of pgenv
that used the old file name style.

Fixed a few problems related to `config` when invoked without any specific
version, that was supposed to translate into `default`.

Readme updated.

README.md

@@ -556,15 +556,15 @@ program will work against the currently active version of PostgreSQL.
 In order to start with a default configuration, use the `write` subcommand:
 
     $ pgenv config write default
-    pgenv configuration file ~/.pgenv/.pgenv.conf written
+    pgenv configuration file ~/.pgenv/.pgenv.default.conf written
 
 A subsequent `show` displays the defaults:
 
 ``` sh
 $ pgenv config show default
 # Default configuration
 # pgenv configuration for PostgreSQL
-# File: /home/luca/git/misc/PostgreSQL/pgenv/.pgenv.conf
+# File: /home/luca/git/misc/PostgreSQL/pgenv/.pgenv.default.conf
 # ---------------------------------------------------
 # pgenv configuration created on mer 12 set 2018, 08.35.52, CEST
 
@@ -628,7 +628,7 @@ loss of configuration:
 
    $ pgenv config delete
    Cannot delete default configuration while version configurations exist
-   To remove it anyway, delete ~/.pgenv/.pgenv.conf.
+   To remove it anyway, delete ~/.pgenv/.pgenv.default.conf.
 
 The `delete` subcommand deletes both the configuration file and its backup
 copy. The `pgenv remove` command also deletes any configuration for the

bin/pgenv

@@ -269,7 +269,7 @@ pgenv_guess_postgresql_version() {
         default)
             # there is no default version, this is useful only
             # for specifying the 'default configuration'
-            v=""
+            v="default"
             ;;
         latest|earliest)
             # must guess!
@@ -383,7 +383,7 @@ pgenv_current_postgresql_version(){
 # file is provided.
 pgenv_configuration_file_name(){
     local v=$1
-    local PGENV_DEFAULT_CONFIG_FILE="${PGENV_ROOT}/.pgenv.conf"
+    local PGENV_DEFAULT_CONFIG_FILE="${PGENV_ROOT}/.pgenv.default.conf"
 
     if [ ! -z "$v" ]; then
         echo "${PGENV_ROOT}/.pgenv.$v.conf"
@@ -1284,10 +1284,16 @@ EOF
     config|configuration)
         action=$2
 
-        # understand the version the user wants to configure
-        v=$( pgenv_guess_postgresql_version $3 $4 )
+
+        if [ -z "$3" ]; then
+            v="default"
+        else
+            # understand the version the user wants to configure
+            v=$( pgenv_guess_postgresql_version $3 $4 )
+        fi
+
         # get the currently in use if none specified
-        if [ -z "$v" ]; then
+        if [ -z "$v"  ]; then
             pgenv_exit_if_no_postgresql_in_use
             v=$( pgenv_current_postgresql_version )
         fi