Introduce exec_file_find

Introduce exec_file_find

This commit adds a new function, exec_file_find, which computes the
full pathname of the main executable in much the same way solib_find
does for pathnames of shared libraries.  The bulk of the existing
solib_find was moved into a new static function solib_find_1, with
exec_file_find and solib_find being small wrappers for solib_find_1.

gdb/ChangeLog:

	* solist.h (exec_file_find): New declaration.
	* solib.c (solib_find_1): New function, factored out from...
	(solib_find): ...here.
	(exec_file_find): New function.