tree-wide: Fix constness issues with newer glibc (#39896)

Latest glibc uses _Generic to have strstr() and other functions return
const char* or char* based on whether the input is a const char* or a
char*. This causes build failures as we previously always expected a
char*.

Let's fix the compilation failures and add our own macros similar to
glibc's to have string functions that return a mutable or const pointer
depending on the input.

diff --cc src/basic/time-util.c
Simple merge