doc: Trivial whitespace fixes

Always use tabs instead of spaces for indentation in documentation
files.
gprs-context: print reason for ppp disconnect
gprs: fix bearer dbus signal interface

According to the documentation the data bearer property should be
located on interface org.ofono.ConnectionManager.
util: Make sure to handle special empty strings

Some SIMs contain an EFspn with the contents all set to 'filler'
characters, e.g. 0xFF.  We mistakenly do not handle these strings
correctly.

Aug  8 11:40:00 mx31tt01 daemon.info ofonod[622]: Aux: >
AT+CRSM=176,28486,0,0,17\r
Aug  8 11:40:00 mx31tt01 daemon.info ofonod[622]: Aux: < \r\n+CRSM:
144,0,FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF\r\n\r\nOK\r\n
Aug  8 11:40:00 mx31tt01 daemon.debug ofonod[622]:
drivers/atmodem/sim.c:at_crsm_read_cb() crsm_read_cb: 90, 00, 17
Aug  8 11:40:00 mx31tt01 daemon.debug ofonod[622]:
src/simfs.c:sim_fs_op_read_block_cb() bufoff: 0, dataoff: 0, tocopy: 17
Aug  8 11:40:00 mx31tt01 daemon.err ofonod[622]: EFspn read
successfully, but couldn't parse
unit: Add a unit test for empty sim strings
sim: add function to get password type
sim: implement function to get password type
sim: support PIN retry counters for telit modems
AUTHORS: Mention August's contributions
sim: Fix warning

drivers/atmodem/sim.c: In function ‘at_pct_cb’:
drivers/atmodem/sim.c:872:2: error: ISO C90 forbids mixed declarations
and code
sim: Minor formatting nitpicks
gprs: data bearer for telit with PSNT

Telit neither supports '+CPSB' nor reports the data bearer through
'+CGREG'. It has its own +PSNT command.
telit: move gprs atom into post sim phase
telit: Fix 80 character per line violation
telit: pass vendor at sim atom creation
mbm: Remove spurious ';'
mbm: Fix crash from processing multiple *EMRDY

Sometimes we see the *EMRDY notification before we query, and trigger
initialization procedures twice.
dundee: add timeout to close stalled ppp handshake

If something goes wrong the PPP handshake can stall, then we just add a
timer to return an error after a certain timeout (15 seconds).
dundee: explicit shutdown the bluetooth file descriptor

If we do not shut it down the fd can remain opened. This make impossible
to try a re-connect: busy is returned in this case.

We call shutdown here to make sure that the link is always closed.
dundee: disconnect bluetooth link in case dial fails

If the ATD*99# dial fails for some reason, NO CARRIER, for example, we
need to disconnect the bluetooth link and do further clean up on
disconnect_callback().