Fix warnings

reqname, bindaddr, request_addr, desthost and orighost to be exhaustive.

This function is used when USE_VFORK is unset.

Changing checkusername()'s username argument into char * as well.

Data is usually represented as "unsigned char *" like genrandom().

error: ‘noreturn’ function does return [-Werror]

abort() is a noreturn function while raise() is not.

And because crypt_argchk() is flagged as __attribute__(noreturn), abort()
appears to be a better condidate.

This compilation warning has probably been introduced by commit
1809f741.

buf_incrwritepos() and mp_to_unsigned_bin() functions use k_size as signed
integer argument.

k_size is also used in an assertion that compared it to curve_size which is
a signed long.

Only buf_getwriteptr() is using k_size as unsigned.

So it safe to use it as signed.

Instead of declaring an "unused-variable" or "unused-but-set-variable"
structure.

This avoid unexpected broken configurations when using something like:
$ ./configure CFLAGS="-Wall -Wextra -Werror"
It causes AC_COMPILE_IFELSE to fail and thus leading to a mismatch
configuration (because the CFLAGS are passed to the macro).