stop using ptrdiff_t
kill a Clang 3.2 scan-build workaround (completely; tested by valgrind and
gcc-snapshot’s sanitisers to never be a problem)
• Build.sh: fix NSIG detection for gcc-snapshot
• all: bump version to R50-current; add more comments; whitespace
• all: remove all mkssert(); we’ll do full re-runs of scan-build and,
  hopefully, Coverity Scan/Prevent
• check.t: fix a testcase (sed could exit false, but we don’t care)
• eval.c: fix tilde_ok data type (only unsigned may shl constantly)
• exec.c: fix shebang buf array accesses to always go via uint8_t *