Needed by TI libc for use with add_device

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If the path starts with /, do not drop it.
There are no directories on SLFS, but use of /pretend/paths/to/files.txt
is common. What we do drop is the ./ prefix (added by mongoose when document_root=.)

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`MG_ENABLE_...` macros are not yet renamed because it involves some
logic change: e.g.  instead of defining `MG_ENABLE_MQTT`, one should
define `MIOT_ENABLE_MQTT`, which should imply `MG_ENABLE_MQTT` as well.

Macro refactoring is going to be done in another PR.

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It wasn't checked for `NULL`, and on CC3200 NULL dereferencing addresses
doesn't cause a crash, so it worked by pure luck: ctx->ssl_key was 0.
After `mg_` to `miot_` refactoring it's not the case anymore (presumably
because linker arranged objects in a different order), so this bug shown
up.

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in light of the root cause discovered by rojer
and addressed in cesanta/dev#5882 for the http connection code path.

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CC3200 (TI's C library) doesn't like it

https://e2e.ti.com/support/development_tools/compiler/f/343/t/554883

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Digital signature key usage is required for the key to be used for (EC)DHE.

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Currently only OpenSSL API impl, but the plan is to add more.

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