Parameter checking: simply return on error, as is the idiom

if zend_parse_parameters() fails, it will print an error message saying
what kind of error occurred (maybe not enough parameters, maybe
parameters of the wrong type), and populate the return value with
`false`. What all the other extensions do in this situation, and what is
documented in the Oreilly PHP book, is to just return.

The current behavior is certainly wrong, because it will print a message
about wrong parameter counts regardless of what the real error is.
smbc_open: add open flags support just like fopen
smbclient_write: make write length optional just like fwrite

No need to pass a third argument to the function with the length of
string to write, when we already know the length of the string.
smbclient_unlink: recognize the EBUSY error

Encountered this error code (undocumented in the libsmbclient header)
when trying to unlink a file that was still open.
README: convert to Markdown, add function descriptions