The Linux kernel suppresses some ICMP error messages by default for
UDP sockets. This commit enables full ICMP error reporting,
hopefully resulting in faster failover to working name servers.
This commit enhances the stub resolver to reload the configuration
in the per-thread _res object if the /etc/resolv.conf file has
changed. The resolver checks whether the application has modified
_res and will not overwrite the _res object in that case.
The struct resolv_context mechanism is used to check the
configuration file only once per name lookup.
struct resolv_context objects provide a temporary resolver context
which does not change during a name lookup operation. Only when the
outmost context is created, the stub resolver configuration is
verified to be current (at present, only against previous res_init
calls). Subsequent attempts to obtain the context will reuse the
result of the initial verification operation.
struct resolv_context can also be extended in the future to store
data which needs to be deallocated during thread cancellation.
The RES_F_* constants are only used with the private _res._flags
member. RES_EXHAUSTIVE is unused. The removed function
declarations refer to functions not actually exported by glibc,
so they are unusable by applications.
