Bugfix: fix buffer overflow with comba sqr

The comba sqr code does not check the maximum bounds of fp_int; eg:
if you invoke fp_sqr_comba_20, it will write 40 digits to the
destination even if FP_SIZE < 40. This is correct for achieving high
speeds, but it means that it is the caller's responsibility to check for
such overflows.

fp_sqr.c only checks for numeric overflows (a->used * 2 >= FP_SIZE)
though. This means that if you call fp_sqr() with a small number (say
1), and your FP_SIZE is 10, and you have enabled a fp_sqr_comba_8, it
will overflow your buffer by writing 16 digits.

Since the exact subset of active comba multipliers/sqrs are up to the user
(in tfm.h), we fix the code never to invoke them if they can cause
overflows.
Fix for buffer overflows in fp_mul.
Revert "Regenerate sqr comba files with bugfix"

This reverts commit ab4374299c46150f6850fb56b524d044134f97c8.
Revert "Bugfix: clear the exceeding destination digits."

This reverts commit 50f587c36f4cb673c7fb79adf453b7af0c6e8ab0.
trim trailing spaces
minor fixes to really reproduce original files
regenerate fp_sqr_comba_small_set.c
Merge remote-tracking branch 'rasky/bugfix_sqr_bufferoverflow'
trim trailing spaces/clean-up
fix compiler warnings
Realign mult generators with existing files in repo.

NB: fp_mul_comba32() is hand-optimized and has to be hand-crafted after
re-generation
change DIGIT_BIT define to be able to test e.g. FP_SIZE in a macro
fixup 3f7a1dca3bf60ce72284499e3cd20952d6845179
implement 50f587c36f4cb673c7fb79adf453b7af0c6e8ab0 as proposed by tom
also patch fp_mul() as proposed by tom on the ML [1]

[1] https://groups.google.com/forum/#!topic/libtom/MdbS1vcLhCU
sqr generator: update preamble not to compile the code if it's not required
mul generator: update preamble not to compile the code if it's not required
Regenerate mul comba files
Regenerate sqr comba files
fix clobber list for x86-64 assembler