AcpiSrc: Add a local version of strlwr() to prevent header issues.

Some systems implement this, others do not, so it is safest to
just have a local version (AsStrlwr).
Generic Unix OSL: Use a buffer to eliminate multiple vfprintf()s.

Some compilers/clibs modify the arg pointer parameter to vfprintf
making it difficult to call it twice in the AcpiOsVprintf function.
Use a local buffer to workaround this issue. Does not affect the
Windows OSL since the win c library does not modify the arg
pointer. Chao Guan, Bob Moore.
Add a comment to the cygwin include file concerning vsnprintf.

The prototype for this function may not be implemented for
older versions of cygwin.