add functions for working with strings
some functions are implemented directlu in strutil.c, others have ascii, 8bit
or utf-8 variant. (8bit means singlebyte encodings, where all characters have
width one). Mc autodetects terminal encoding at start and chooses right
variant. If does not know terminal encoding, chooses ascii variant.
contains functions:
1. for translation strings and growing strings
2. for working with characters (next char, prev char, length in
characters, isspace, isalnum, ...)
3. prepeare for display, replace invalid characters with questionmark,
unprintable with dot, left / right / center align
4. comparing strings
in future all string function from util should be moved into strutil, some
function from util have new variant in strutil.
