[MTX-3785] mPower R.5.4: GP-971 - Device shall implement file type restrictions for all “upload” sections on the UI

See merge request !3

LoRaWAN components: LoraLicense (.lic), LoraFotaFile (.bin), LoraMulticast (.bin for Binary, .txt for Hexadecimal and Base64), LoraGroup (.json) validated by file extension.

[MTX-3669] mPower Release 5.4: alertservice and libmts shall print log messages to stderr

See merge request !2

[MTX-3311] Execute system commands without invoking system shell

See merge request !1

Added a missed ```child``` parameter to the list of ```executeBackground``` params in docs

Changes after a code review:

- fixed documentation for the closeBackground function
- renamed ```app``` parameter to ```cmd```

Added several new methods to the MTS::System class:

- ```executeBackground``` - popen alternative, creates a pipe with the specified
  mode and spawns a new process with specified arguments;
- ```closeBackground``` - pclose alternative, closes parent side of the pipe and
  waits for the child process to finish its execution;
- ```execute``` - MTS::System::cmd alternative that allows to spawn child
  processes directly without invoking system shell.