Merge topic branch t/overflow into master

Merge topic branch t/overflow into master

This series implements a new memory allocation API which checks
for overflows. The first part of the series just renames the main
allocation functions. Later patches in the series implement allocators
which take two size_t arguments (like calloc(3)) and check whether the
multiplication overflows by employing the __builtin_mul_overflow()
primitive supported by gcc and clang. This requires us to bump the
lowest supported gcc and clang version.

* refs/heads/t/overflow:
  build: Compile with -ftrapv.
  string: Introduce arr_zalloc().
  string: Introduce arr_alloc().
  string: Introduce arr_realloc() and check for integer overflow.
  string: Rename para_calloc() -> zalloc().
  string: Rename para_malloc() -> alloc().
  string: Overhaul para_strdup().