Fix DNS mirror forward redirects Issue: gmt-admin #297 Incorrect DNS settings for mirrors

[ES-Solar]

Problem

Related to GenericMappingTools/gmt-admin#297

Alias server names such as china and australia expand to Hover DNS
URL-forwards (e.g., http://china.generic-mapping-tools.org) that redirect
all requests to the mirror root — stripping the file path in the process.
This caused every file download to fail: GMT received an HTML directory
listing instead of the requested grid or cache file.

Fix

gmtremote_resolve_redirect() in gmt_remote.c follows the redirect chain
once at session start to discover the real mirror base URL
(e.g., https://mirrors.ustc.edu.cn/gmtdata). gmt_dataserver_url() caches
the result for the process lifetime and uses it for all subsequent file URL
construction. Direct http:// URLs and servers with no redirect are
unaffected.

Test

Added test/gmt_remote/dataserver_alias.sh which forces GMT_DATA_SERVER=oceania,
clears the local cache, downloads @earth_relief_15m_g.grd, and verifies the
grid region with gmt grdinfo -I-.

[joa-quim]

Use the earth_relief_01d_p.grd grid instead. Much smaller.

[joa-quim]

Also, don't you have permission writes to master? Because PRs submitted to master are much easier for us to test.

[ES-Solar]

I updated the test to use the smaller grid.

was used to the updating my fork and thought that was the preferred workflow. I'll change that for the next PR I contribute.