mcpp: fix set_encoding() to understand C.UTF-8

[gt7-void]

Function set_encoding() finds the encoding name by splitting a locale of the form en_US.UTF-8 at the period, but only if the leading part is exactly 5 characters long.

$ LC_ALL=C.UTF-8 mcpp /dev/null
/dev/null:0: warning: Unknown encoding: C.UTF-8
    #line 1 "/dev/null"

This patch changes the behaviour to split at the first period instead.

This will also fix a warning in xrdb:

$ LC_ALL=C.UTF-8 xrdb -merge /dev/null
/dev/null:0: warning: Unknown encoding: C.UTF-8