freetdm: ftmod_libpri + ftmod_zt: some DAHDI drivers return an error of ELAST (500) on read()/write() to indicate there are events pending.

Fixup zt_read() to handle this case correctly and rework ftmod_libpri's read wrapper function to not fail when the read function returns zero bytes.

NOTE: zt_write() has not been changed (some better way to handle these events is needed then)

This should fix these log messages:

   [WARNING] ftdm_io.c:3561 [s1c16][1:16] raw I/O read filed
   [CRIT] lpwrap_pri.c:125 span 1 D-READ FAIL! []
   [CRIT] lpwrap_pri.c:157 span 1 D-WRITE FAIL! []
   [ERR] ftmod_libpri.c:131 Short write: -1/6 (Unknown error 500)
Signed-off-by: Stefan Knoblich <s.knoblich@axsentis.de>