varnishd: truncate thread name on Linux

[asadsa92]

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.

[nigoroll]

merged per bugwash decision