On Linux, threads can not have name longer than 15 bytes plus a terminating
'\0' byte:
> PR_SET_NAME (since Linux 2.6.9)
>     Set the name of the calling thread, using the value in the loca‐
>     tion  pointed  to  by  (char  *) arg2.  The name can be up to 16
>     bytes long, including the terminating null byte.  (If the length
>     of  the  string, including the terminating null byte, exceeds 16
>     bytes, the string is silently truncated.)  This is the same  at‐
>     tribute  that can be set via pthread_setname_np(3) and retrieved
>     using pthread_getname_np(3).  The attribute is likewise accessi‐
>     ble via /proc/self/task/tid/comm (see proc(5)), where tid is the
>     thread ID of the calling thread, as returned by gettid(2).

We have until now ignored the return value from pthread_setname_np(), this
is not great as the call then becomes a NOP:
> The pthread_setname_np() function can be used to  set  a
>     unique  name  for  a  thread,  which can be useful for debugging multi‐
>     threaded applications.  The thread name  is  a  meaningful  C  language
>     string, whose length is restricted to 16 characters, including the ter‐
>     minating null byte ('\0').
> [...]
>     ERANGE The  length  of  the string specified pointed to by name exceeds
>         the allowed limit.

This patch truncates long names to 14 characters plus a tilde ('~') character.
Signed-off-by: Asad Sajjad Ahmed <asadsa@varnish-software.com>