Optional init sequence (#59653)

[Raphaël]

# https://savannah.nongnu.org/bugs/?59653

Back in 2011 December (98a87466), after 0.6.31 release, a change was
introduced so that if any of ATZ, ATE1, AT+CMEE=1) returns the error,
fail Initialize()

If find this a bit bold.
I just encountered a "MTT Mini" feature phone which replies an
"ERROR 100" to ATZ (but "OK" to ATE1 and CMEE).

This model also throw an ERROR 100 to CGMI/GMI

I think the init-sequence should be may (optionally) tolerant to buggy
phone AT implementation.

It looks like the usb-modeswitch "quirks" and I tried to make that
optional-init-sequence a flag but couldn't figure how to do that for the
AT pseudo-model so I'd let that to you.

optional-init-sequence.patch
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