Add function to parse RFC822

[thernstig]

Would be nice to add a function that can parse RFC822: https://datatracker.ietf.org/doc/html/rfc822#section-5.1.

It is used in the output of openssl validity dates output a string according to RFC822.

> openssl x509 -dates -noout -in ./ca.crt
notBefore=Apr 16 09:17:18 2024 GMT
notAfter=Mar 23 09:17:48 2124 GMT

RFC 822 says:

     date-time   =  [ day "," ] date time        ; dd mm yy
                                                 ;  hh:mm:ss zzz
     
     day         =  "Mon"  / "Tue" /  "Wed"  / "Thu"
                 /  "Fri"  / "Sat" /  "Sun"
     
     date        =  1*2DIGIT month 2DIGIT        ; day month year
                                                 ;  e.g. 20 Jun 82
     
     month       =  "Jan"  /  "Feb" /  "Mar"  /  "Apr"
                 /  "May"  /  "Jun" /  "Jul"  /  "Aug"
                 /  "Sep"  /  "Oct" /  "Nov"  /  "Dec"
     
     time        =  hour zone                    ; ANSI and Military
     
     hour        =  2DIGIT ":" 2DIGIT [":" 2DIGIT]
                                                 ; 00:00:00 - 23:59:59
     
     zone        =  "UT"  / "GMT"                ; Universal Time
                                                 ; North American : UT
                 /  "EST" / "EDT"                ;  Eastern:  - 5/ - 4
                 /  "CST" / "CDT"                ;  Central:  - 6/ - 5
                 /  "MST" / "MDT"                ;  Mountain: - 7/ - 6
                 /  "PST" / "PDT"                ;  Pacific:  - 8/ - 7
                 /  1ALPHA                       ; Military: Z = UT;
                                                 ;  A:-1; (J not used)
                                                 ;  M:-12; N:+1; Y:+12
                 / ( ("+" / "-") 4DIGIT )        ; Local differential
                                                 ;  hours+min. (HHMM)