Refactor charset plugins a bit and add CP437 module.

Refactor charset plugins a bit and add CP437 module.
Now all 8-bit charsets with gaps (not all symbols defined) could be produced through
one macro -- SMB_GENERATE_CHARSET_MODULE_8_BIT_GAP(CHARSETNAME) within source file
with three charset tables. Full source code for such modules can be generated by
source/script/gen-8bit-gap.sh script which was taken from GNU libc and changed slightly
to follow our data types and structure.
(This used to be commit 37042c7bc0f349370e93e4bed37d8fa371013247)