add an option to re-compute the CRC value of a modified export file

[PeterPawn]

• this may only make sense for the C implementation, because dissection of an export file with shell commands is time-consuming again and the re-composition isn't really faster

[PeterPawn]

• partially done with https://github.com/PeterPawn/decode_passwords/commit/38396df2c1f0e000a9ea1bb0c1a5f7d7a044bbd9
• needs to be integrated into 'decode_export' applet yet

[PeterPawn]

finally completed with https://github.com/PeterPawn/decode_passwords/commit/519c8664b8508fb7da1fa0bb476879cada1b009f