sms_tool: make USSD response decoding robust

[dormancygrace]

What changed

• replace unbounded, one-line sscanf() parsing of +CUSD with a bounded parser
• accumulate split/multiline CUSD responses before parsing
• print already-decoded text returned by modems instead of treating it as hex
• conservatively recognize UCS2 payloads when modem firmware reports GSM-7
• preserve -c as an authoritative manual override
• fix the DCS bit-test precedence bug and bound UCS2-to-UTF-8 output

Why

Issue #13 contains a valid UCS2 response reported with DCS 7 by the modem.
The old code decoded it as packed GSM-7 unless the user supplied -c 2.
The USSD part of #1 also remained limited to one-line responses, and the old
unbounded scans could overflow the 320-byte payload buffer (the sample from
#13 is already 496 hex characters).

This completes the unresolved USSD portions of #1 and #13 while keeping the
existing manual workaround available.

Validation

• exact response from USSD reply not decoding? #13 decodes to the expected Swedish text without -c
• -c 2 still decodes the same response
• split plain-text response based on Decoding issues with operator that doesn't use numbers in FROM (Orange.pl) #1 is reassembled and printed correctly
• native release build
• AddressSanitizer and UndefinedBehaviorSanitizer integration runs
• MIPS 24Kc/musl OpenWrt cross-compilation
• git diff --check