svf_read_command_from_file() function

Reads a line from the file handle (without the trailing NL or CRNL) and returns it in a stringbuf. Returns NULL if no characters are read before EOF or error. Note that the character count will *not* be correct for lines containing utf8 sequences. Do not rely on the character count.

Creates a channel for fh/fd/filename. If fh is not NULL, uses that as the channel (and sets AIO_KEEPOPEN). Otherwise fd must be >= 0, in which case it uses that as the channel. hdlfmt is a sprintf format for the filehandle. Anything with %ld at the end will do. mode is used for open or fdopen. Creates the command and sets the name as the current result. Returns the AioFile pointer on sucess or NULL on failure (only if fdopen fails).