connectors/ssh: let autoconf find default remote shell

Problem: default remote shell is hardwired in the ssh
connector source to "/usr/bin/rsh".  On systems that don't
install "mrsh" (TOSS) and don't use debian alternatives,
/usr/bin/rsh is not a symlink to the preferred remote shell.

Have autoconf perform the search for a suitable remote
shell program, and put the results in HAVE_SSH, then
use that in the ssh connector source.

Configure searches in the current path for "rsh", then "ssh",
then falls back to /usr/bin/rsh as before if none found.

configure.ac

@@ -107,6 +107,10 @@ else
             [deepbind is unsupported with asan, musl and so-forth])
 fi
 
+# N.B. /usr/bin/rsh is a symlink to preferred remote shell on some systems
+AC_PATH_PROGS(SSH, [rsh ssh], [/usr/bin/rsh])
+AC_DEFINE_UNQUOTED([PATH_SSH], "$SSH",
+          [Define remote shell program to be used by the ssh:// connector])
 
 LT_INIT
 AC_PROG_AWK

src/connectors/ssh/ssh.c

@@ -28,8 +28,6 @@
 #include "src/common/libutil/popen2.h"
 #include "src/common/libutil/fdutils.h"
 
-static const char *default_ssh_cmd = "/usr/bin/rsh";
-
 #define CTX_MAGIC   0xe534babb
 typedef struct {
     int magic;
@@ -361,7 +359,7 @@ flux_t *connector_init (const char *path, int flags)
     c->magic = CTX_MAGIC;
 
     if (!(c->ssh_cmd = getenv ("FLUX_SSH")))
-        c->ssh_cmd = default_ssh_cmd;
+        c->ssh_cmd = PATH_SSH;
     if (argz_add (&c->ssh_argz, &c->ssh_argz_len, c->ssh_cmd) != 0)
         goto error;
     if (parse_ssh_port (c, path) < 0) /* [-p port] */