Complete revamp of the pathexec functions

- pathexec_run is now called exec_ae
    a for provided file name (default: argv[0])
    e for provided envp (default: environ)
- pathexec is now called mexec. m for merge environment. Option letters are:
    a for provided file name (default: argv[0])
    e for provided envp (default: environ)
    f for provided envp *and* length of the envp
    m for provided modif string plus its length (the length is always needed
because the modifs are null-terminated)
    n for provided modif string, length *and* number of modifs
- functions have a foo0 version for _exit(0) when argv[0] is null
- functions have a xfoo version to die if the exec fails
- and a xfoo0
- Compatibility #defines and #includes are there until the next major bump

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