Log file:line instead of function; add cs_log_set_file_level()

cs_log_set_filter() is removed in favor of cs_log_set_file_level() which
allows setting log verbosity of individual files and/or lines.

E.g.: `mg_=1,mjs=1,=4` - level for for everything except mjs and mongoose.

We use the fact that we are usually pretty careful with our file names and use prefixes consistently.

In mos, `debug.file_level` sets the option on boot and `Sys.SetDebug` is updated to parse `file_level` option to change it at runtime.

CL: Log file:line instead of function; add cs_log_set_file_level()

PUBLISHED_FROM=49f18d4d0d3dd224865129a4f8fe6cccd1d9bc12

include/mongoose.h

@@ -2647,6 +2647,8 @@ const char *mg_next_comma_list_entry(const char *list, struct mg_str *val,
 
 /*
  * Like `mg_next_comma_list_entry()`, but takes `list` as `struct mg_str`.
+ * NB: Test return value's .p, not .len. On last itreation that yields result
+ * .len will be 0 but .p will not. When finished, .p will be NULL.
  */
 struct mg_str mg_next_comma_list_entry_n(struct mg_str list, struct mg_str *val,
                                          struct mg_str *eq_val);