maint: change "can not" to "cannot"

maint: change "can not" to "cannot"

But do not change the occurrences in maintain.texi or in
build-aux/po/Makefile.in.in, which I presume comes from gettext.
* doc/gnulib-tool.texi: s/can not/cannot/
* doc/posix-functions/accept.texi (accept): Likewise.
* doc/posix-functions/socket.texi (socket): Likewise.
* lib/mbrtowc.c: Likewise.