Replace unsetenv() and setenv() with older putenv().

Replace unsetenv() and setenv() with older putenv().

Solaris 8 doesn't have the newer unsetenv() and setenv()
functions, so replace them with putenv().  The one use of
unsetenv() in fsck-cache.c now sets GIT_ALTERNATE_OBJECT_
DIRECTORIES to the empty string.  Every place that var
is used, NULLs are also replaced with empty strings, so
it's ok.

Signed-off-by: Jason Riedy <ejr@cs.berkeley.edu>