Allow passing fd == NULL to exec_file_find and solib_find

Allow passing fd == NULL to exec_file_find and solib_find

This commit allows NULL to be passed as the int *fd argument
to exec_file_find and solib_find to simplify use cases where
the caller does not require the file to be opened.

gdb/ChangeLog:

	* solib.c (solib_find_1): Allow fd argument to be NULL.
	(exec_file_find): Update comment.
	(solib_find): Likewise.
	* exec.c (exec_file_locate_attach): Use NULL as fd
	argument to exec_file_find to avoid having to close
	the opened file.
	* infrun.c (follow_exec): Likewise.