Fix some slow uses and misuses of strcat.

Fix some slow uses and misuses of strcat.

* doc.c (get_doc_string):
* gtkutil.c (get_utf8_string):
* xsmfns.c (x_session_initialize):
Avoid recomputation of string length.
* ftfont.c (ftfont_spec_pattern):
* xfns.c (xic_create_fontsetname):
Don't assume output buffer is initially zero.