Reduce dependency on utils.h - new malloc wrapping scheme.

Remove xmalloc, xcalloc, xrealloc and xstrdup from utils.h and revert calls
to the normal malloc, calloc, realloc, strdup.  Add new files malloc_wrap.[ch]
with the wrapper functions.  malloc_wrap.h #defines malloc etc. to the
wrapper, but only if USE_MALLOC_WRAPPERS has been defined.

Put #include "malloc_wrap.h" in any file that uses *alloc or strdup.  This
is also in a #ifdef USE_MALLOC_WRAPPERS ... #endif block to make using the
wrappers optional.  Add -DUSE_MALLOC_WRAPPERS into the makefile so they
should normally get added.

This is an improvement on the previous method as we now don't need to
worry about stray function calls that were not changed to the wrapped version
and the code will still work even if the wrapping is disabled.

Other possible methods of doing this are using malloc_hook (glibc-specific),
adding -include malloc_wrap.h to the gcc command-line (somewhat
gcc-specific) or making our own malloc function and using dlopen (scary).
This way is probably the most portable.
Removed more dependencies on utils.h

bamlite.c now includes some wrappers for gzopen/gzread/gzclose that print
messages when errors occur.  They do not attempt to quit the program but
pass on the return code.  bwaseqio.c now checks the return codes from
bam_open, bam_close and bam_read1.

Code in bwt_gen.c now checks for IO errors itself instead of using the
wrappers.  A benefit of this is it can now say which file had a problem.

Removed call to err_fatal_simple in is_bwt and unnecessary inclusion of
malloc_wrap.h in ksw.h.