[WIP/RFC] Adapt os module to the style guide

This PR tries to fix the style of the os module.
What is done so far:
* `mch_*` functions are renamed to `os_*`
* `/*...*/` are replaced with `//...` comments
* some other clint warnings are fixed (like too long lines, wrong parentheses, etc.)

Remaining `clint.py` errors:
```
$ python clint.py src/os/*
src/os/env.c:0:  No copyright message found.  You should have a line: "Copyright [year] <Copyright Owner>"  [legal/copyright] [5]
Done processing src/os/env.c
src/os/event.c:0:  No copyright message found.  You should have a line: "Copyright [year] <Copyright Owner>"  [legal/copyright] [5]
Done processing src/os/event.c
src/os/event.h:0:  No copyright message found.  You should have a line: "Copyright [year] <Copyright Owner>"  [legal/copyright] [5]
src/os/event.h:1:  #ifndef header guard has wrong style, please use: SRC_OS_EVENT_H_  [build/header_guard] [5]
src/os/event.h:15:  #endif line should be "#endif  // SRC_OS_EVENT_H_"  [build/header_guard] [5]
Done processing src/os/event.h
src/os/fs.c:0:  No copyright message found.  You should have a line: "Copyright [year] <Copyright Owner>"  [legal/copyright] [5]
src/os/fs.c:124:  Do not use variable-length arrays.  Use an appropriately named ('k' followed by CamelCase) compile-time constant for the size.  [runtime/arrays] [1]
Done processing src/os/fs.c
src/os/input.c:0:  No copyright message found.  You should have a line: "Copyright [year] <Copyright Owner>"  [legal/copyright] [5]
Done processing src/os/input.c
src/os/input.h:0:  No copyright message found.  You should have a line: "Copyright [year] <Copyright Owner>"  [legal/copyright] [5]
src/os/input.h:1:  #ifndef header guard has wrong style, please use: SRC_OS_INPUT_H_  [build/header_guard] [5]
src/os/input.h:19:  #endif line should be "#endif  // SRC_OS_INPUT_H_"  [build/header_guard] [5]
Done processing src/os/input.h
src/os/mem.c:0:  No copyright message found.  You should have a line: "Copyright [year] <Copyright Owner>"  [legal/copyright] [5]
Done processing src/os/mem.c
src/os/os.h:0:  No copyright message found.  You should have a line: "Copyright [year] <Copyright Owner>"  [legal/copyright] [5]
src/os/os.h:1:  #ifndef header guard has wrong style, please use: SRC_OS_OS_H_  [build/header_guard] [5]
src/os/os.h:24:  #endif line should be "#endif  // SRC_OS_OS_H_"  [build/header_guard] [5]
Done processing src/os/os.h
src/os/time.c:0:  No copyright message found.  You should have a line: "Copyright [year] <Copyright Owner>"  [legal/copyright] [5]
Done processing src/os/time.c
src/os/time.h:0:  No copyright message found.  You should have a line: "Copyright [year] <Copyright Owner>"  [legal/copyright] [5]
src/os/time.h:1:  #ifndef header guard has wrong style, please use: SRC_OS_TIME_H_  [build/header_guard] [5]
src/os/time.h:10:  #endif line should be "#endif  // SRC_OS_TIME_H_"  [build/header_guard] [5]
Done processing src/os/time.h
src/os/users.c:0:  No copyright message found.  You should have a line: "Copyright [year] <Copyright Owner>"  [legal/copyright] [5]
src/os/users.c:60:  Consider using getpwuid_r(...) instead of getpwuid(...) for improved thread safety.  [runtime/threadsafe_fn] [2]
src/os/users.c:80:  Consider using getpwnam_r(...) instead of getpwnam(...) for improved thread safety.  [runtime/threadsafe_fn] [2]
Done processing src/os/users.c
Total errors found: 22
```

Most of the errors are wrong (header guards and copyright warnings). These are fixed by #409.
Any thoughts on the last two errors (use `getpw[uid|nam]_r` instead of `getpw[uid|nam]`) are appreciated.
Also feel free to be pedantic with this PR. Surely there are lots of style issues which can be solved in this module.