Replace timegm

[tkob]

This PR adds mlr_timegm function and replace timegm with mlr_timegm. (Issue #20)

[johnkerl]

Nice, thank you!

[jirutka]

This is a quite bad improvement – mlr_timegm is not thread-safe!

[johnkerl]

@jirutka can you tell me how threads are involved in Miller?

[jirutka]

I don’t know. I was just searching for a portable timegm replacement when I came across this PR. Even if you currently don’t use threads in the project, writing code like this without any warning/comment that it’s not thread-safe is generally a pretty bad practice.

[johnkerl]

@jirutka good call!

[johnkerl]

@jirutka Miller was ported from C to Go as of 2022, and the C code has been removed from the main branch of Miller. I'll add a comment on a branch ...