Missing and incorrect documentation and specs for atomvm:posix_* functions

[UncleGrumpy]

The documentation for atomvm:posix_write/2 states:

Write data to a file. Files are open non-blocking. ˋatomvm:posix_select_write/3` can be used to determine if the file can be written.

The function ˋatomvm:posix_select_write/3` does not seem to be implemented in posix_nifs.c

Several other nifs in posix_nifs.c are not documented, since these are not standard functions from an OTP module users will have no way of knowing they exist. These functions include:

• atomvm:posix_closedir/1
• atomvm:posix_mkfifo/2
• atomvm:posix_select_read/2
• atomvm:posix_select_write/2
• atomvm:posix_select_stop/2
• atomvm:posix_unlink/1

Also in the atomvm module atomvm:file_get_cwd/0 is lacking documentation.

As well as making the existence of these functions known to users, specs would be beneficial for dialyzer.