Cleanup swapm routine

[jayeshkrishna]

Cleanup the pio_swapm routine,

• Use collective based on whether collective option is chosen in the flow control option (rather than the number of maximum pending requests)
• Use the number of maximum pending requests to determine whether to use blocking routines

Also check if the C version (Scorpio) of the function deviates from the Fortran version (Scorpio classic) in any other way.