add locale framework

this commit adds non-stub implementations of setlocale, duplocale,
newlocale, and uselocale, along with the data structures and minimal
code needed for representing the active locale on a per-thread basis
and optimizing the common case where thread-local locale settings are
not in use.

at this point, the data structures only contain what is necessary to
represent LC_CTYPE (a single flag) and LC_MESSAGES (a name for use in
finding message translation files). representation for the other
categories will be added later; the expectation is that a single
pointer will suffice for each.

for LC_CTYPE, the strings "C" and "POSIX" are treated as special; any
other string is accepted and treated as "C.UTF-8". for other
categories, any string is accepted after being truncated to a maximum
supported length (currently 15 bytes). for LC_MESSAGES, the name is
kept regardless of whether libc itself can use such a message
translation locale, since applications using catgets or gettext should
be able to use message locales libc is not aware of. for other
categories, names which are not successfully loaded as locales (which,
at present, means all names) are treated as aliases for "C". setlocale
never fails.

locale settings are not yet used anywhere, so this commit should have
no visible effects except for the contents of the string returned by
setlocale.
consolidate *_l ctype/wctype functions into their non-_l source files

the main practical purposes of this commit are to remove a huge amount
of clutter from the src/locale directory, to cut down on the length of
the $(AR) and $(LD) command lines, and to reduce the amount of space
wasted by object file headers in the static libc.a. build time may
also be reduced, though this has not been measured.

as an additional justification, if there ever were a need for the
behavior of these functions to vary by locale, it would be necessary
for the non-_l versions to call the _l versions, so that linking the
former without the latter would not be possible anyway.
consolidate str[n]casecmp_l into str[n]casecmp source files

this is mainly done for consistency with the ctype functions and to
declutter the src/locale directory.
properly pass current locale to *_l functions when used internally

this change is presently non-functional since the callees do not yet
use their locale argument for anything.