Perf

[cgzones]

• Annotate mbstowcs_nonfatal() with restrict

  The arguments to our mbrtowc(3) wrapper should not alias, since they also must not for mbrtowc(3).

  Might help some compilers optimizing the code.

• Linux: use fast integer parsing for /proc//stat

  /proc//stat is parsed for every displayed process on every iteration containing several numbers to parse. Use out own implementations since strto(u)l(3) came up during profiling.

  @BenBE what was the reason for adding the parameter maxlen to those functions?

[BenBE]

The reason for the maxlen argument had mostly two aspects:

• Defense in depth regarding avoiding buffer overruns
• Unless the input is all zero, every string above a certain length will overflow.

[BenBE]

@cgzones Do you mind updating this PR?