Let the config parser continue parsing even after encountering an error;
original idea (with now-outdated patch) from Thomas Adam.  Since we now
report where errors exist, this now makes more sense.  Expand a bit on
config file parsing in the cwm(1).

Discussed with a few, including sthen; ok sthen.
Take menu's bwidth into account for position/size; reported by Thomas Adam.
The menu already limits entries with MENU_MAXENTRY, so don't bother
holding a command name limit as well.
Unlike free(3), XFree(3) can't be passed a NULL pointer.
No need to store screen colormap and visual; rather just use the X
macros in the one place they are needed.
Move redundant window attr fetch from maprequest directly into
client_init and perform that X roundtrip only once.