src/useradd.c: E_BAD_NAME: Use a different error code for bad login names

[alejandro-colomar]

Wrappers like adduser(8) want to do their own stuff if the login name is
bad.  For that, they need to be able to differentiate such an error.

Closes: #1103
Suggested-by: @zeha
Cc: @Zugschlus

---

Revisions:

v1b

• Rebase

$ git range-diff alx/master..gh/badname master..badname 
1:  4c084b1d = 1:  d288ade5 src/useradd.c: E_BAD_NAME: Use a different error code for bad login names

v1c

• Rebase

$ git range-diff alx/master..gh/badname master..badname 
1:  d288ade5 = 1:  5b17bc41 src/useradd.c: E_BAD_NAME: Use a different error code for bad login names

v1d

• Rebase

$ git range-diff master..gh/badname shadow/master..badname 
1:  5b17bc41 = 1:  f63fb09e src/useradd.c: E_BAD_NAME: Use a different error code for bad login names

v1e

• Reviewed by @hallyn

$ git range-diff master gh/badname badname 
1:  f63fb09e ! 1:  01ddd7d8 src/useradd.c: E_BAD_NAME: Use a different error code for bad login names
    @@ Commit message
         Closes: <https://github.com/shadow-maint/shadow/issues/1103>
         Suggested-by: Chris Hofstaedtler <[email protected]>
         Cc: Marc 'Zugschlus' Haber <[email protected]>
    +    Reviewed-by: Serge Hallyn <[email protected]>
         Signed-off-by: Alejandro Colomar <[email protected]>
     
      ## src/useradd.c ##

v1f

• Rebase

$ git range-diff master..gh/badname shadow/master..badname 
1:  01ddd7d8 = 1:  00ca2301 src/useradd.c: E_BAD_NAME: Use a different error code for bad login names

[hallyn]

lgtm

[alejandro-colomar]

lgtm

Thanks! I've now formally opened for review. :)